Basically, if you're an idiot, they're the people on the internet that try to aggravate you. This mostly happens to Justin Bieber fans, horny girls and guys on omegle, and idiots in general. Either that, or the troll just has too much time on their hands and trolls any random person for shits and giggles.

A person who posts things on some online community setting that are designed to purposefully assault and offend large numbers of people for no reason other than the Troll's bias against them, hatred for them or personal amusement at annoying them.

Guy 1: Have you been on Encyclopaedia Dramatica?
Guy 2: No!! That place is for Trolls!!
Troll: OMG JOO PEDOPHILE!! U R TEH GAY 1D10T!! G0 DIE JOO FAIL!! LULZ IS GUD ROFLMAO!!
Guy 1: Ah. I see what you mean.